The Ras family of proteins play critical roles in cell proliferation, differentiation, and cell migration in response to extracellular signals. Ras proteins are small membrane-bound GTPases that act as molecular switches, cycling between an inactive GDP-bound state and an active GTP-bound state. In the most well studied Ras-mediated signal transduction pathways, Ras is activated by receptor tyrosine kinases (RTK) through guanine nucleotide exchange factors that promote GTP binding and a change in Ras conformation to an active state (See e.g., McCormick, Nature 363:15 [1993]). GTP-bound Ras then binds to the serine/threonine kinase Raf and recruits it to plasma membrane where it is activated. Once activated, Raf phosphorylates and activates the dual specific kinase MEK, which in turn phosphorylates and activates MAP kinase. Activated MAP kinase (MAPK) is proposed to regulate the activity of multiple targets including transcription factors for various physiological functions (Marshall, Curr. Opin. Genet. Dev., 4:82 [1994]). Although this model for Ras-dependent signal transduction has been heavily studied, there has been almost no development or identification of effectors that regulate Ras signal transduction or that alter the associated cellular and physiological events stimulated by Ras. Little is known about the nature of Ras effectors or the pathways they control (Rubin et al., WO 97/21820 [1997]).
Recent studies using various model systems including biochemical studies in mammalian tissue culture and genetics in C elegans and Drosophila suggest that the RTK-Ras-MAPK-mediated signal transduction pathway is not a simple linear pathway, but is likely part of complicated signal transduction network (Katz, Curr. Opin. Genet. Dev., 7:15 [1997]; Sundaram and Han, Cell 83, 889 [1995]; and Kornfeld, Trends Genet., 13:55 [1997]). Thus, a series of converging and diverging signalling pathways are likely responsible for the diverse cellular responses mediated by Ras. In recent years, several potential Ras effectors in addition to Raf, including PI3 kinase and Ral GDS, have been described (Katz, supra) and are candidates for defining branch points of Ras signalling. However, these effectors cannot account for all of the cellular responses mediated by Ras (See e.g., White et al., Cell 80:533 [1995]) and have not been sufficiently characterized.
Adding to the complexity of the various signaling processes is the collaboratory roles of multiple factors and signaling branches in regulating the output of the signal. The main players of the RTK-Ras-MAPK pathway may be essential elements of a given signaling process, but there are other factors that feed into or out of this pathway that may play important regulatory functions to ensure maximal activity of the pathway and to tighten the regulation of the signal. For example, the ksr genes were identified as suppressors of activated ras in C. elegans and Drosophila (Sundaram and Han, Cell 83:889 [1995]; Kornfeld et al., Cell 83:903 [1995]; and Therrien et al., Cell 83:879 [1995]), however, their biochemical relation to the Ras pathway is still not well understood. In C. elegans, it has been shown that mutations in the ksr-1 gene do not obviously disrupt vulval signal transduction mediated by ras (i.e., a pathway controlled by ras in C. elegans). However, the ksr-1 activity becomes essential when the activity in the main pathway is compromised (Sundaram and Han, 1995, supra; and Kornfeld et al., 1995, supra).
The art is in need of additional regulators of the Ras signal transduction pathways. To gain regulatory control of Ras signaling and its physiological consequences (e.g., effects on cancer), new Ras effectors and their genes need to be identified and isolated. Without such developments, the ability to control Ras-mediated proliferation, differentiation, and cell migration will be severely limited.